Currently, due to the constant growth of vehicles, the number of used tyres has been increasing through years. It is estimated that in Mexico 25 millions of tyres are disposed yearly and that in the U.S. about one tyre per habitant, therefore constituting one of the acutest pollution problems of the last few years, moreover taking into account that storing, destroying or finding a practical use to the large volumes of used tyres represents a great complexity. One main reason for this problem is due to the physicochemical properties the tyres acquire when subjected to vulcanization, making their elimination, recycling or transformation very complicated.
In Mexico, most of the used tyres normally may be found in streets, breeding grounds, open sky garbage dumps and in few cases in landfills. Then, these deposits faster become a health problem, since vermin such as mice and mosquitoes are developed therein.
Recently, the Cement industry has been making use of used tyres as fuel for their furnaces, since tyres are mainly formed by hydrocarbons thus containing a very similar heat capacity to that of fossil fuels employed in this industry, or even higher than that of carbon. Tyres are incinerated at temperatures about 1450 C, which according to said industry, reaches a complete combustion without residues as soot and other pollutants. However, nowadays only a limited amount of tyres for such purposes is allowed to be employed.
Another application for used tyres has been as fillers in highways and bridges slopes, but this use is reduced due to the physical and chemical properties of tyres.
Regarding tyre recycling, there are technologies to recover the main tyre materials form which they are manufactured, in other words, rubber, polymeric and steel fibers and cords. These recovered materials are employed mainly to make new tyres, making carpets, waterproofs, cast steel, rubber and cork agglomerates, shoe soles, etc, in addition, old tyres may be again vulcanized with new rubber. In tyre recycling, same are split, cut or ground to obtain particles, strips, sheets or pellets which may be incorporated in the above mentioned products and processes.
However, recycling is a complicated process due to the particular tyre structure in which the polymeric and steel cords are inside and between the rubber layers, which makes it not easy nor direct to obtain particles having a homogeneous structure.
In order to clearly explain the above, it is convenient to point out that a tyre has a structure that is defined by two main parts: the tread which contacts the pavement and the lateral faces allowing a tyre to be mounted on a rim.
Related to the above, the tread is in turn constituted by an inner layer and an outer layer or tread bottom, the last part making contact with the pavement. The inner layer is formed mainly by a carcass or frame having inside a sealing rubber layer; rubber coated polymeric fiber cords and depending on the type of tyre, balancing belts. On the other hand, the outer layer or tread bottom is essentially comprised by rubber, this outer layer is provided with bars, groves, flutings and shoulders, which form the drawing of the tyre.
Describing in more detail the inner layer, the carcass or frame is conformed in its most inner part by the sealing rubber layer, which acts as an air restrainer because it is airtight. Over the sealing rubber layer are located the cords of polymeric materials, such as rayon, polyester, nylon or steel being covered by rubber in order to avoid any friction therebetween. Finally, in the casing most outer part and over the polymeric or metallic cord layers are optionally located the balancing belts, which main purpose is to provide balance and uniformity to the tread when the tyre is rotating at high speeds, thus contributing to the tyre strength, less wearing, traction and driving. Commonly, the balancing belts are made from steel, which provides strengthening and balancing to the tread without adding much weight to the tyre.
Referring now to the second main part forming the tread, in other words, the outer layer or tread bottom, it may be noted that the grooves provided in said outer layer are designed to let water and residues to run off, cooling the tyre, traction generation, and to prevent a vehicle lateral slipping. In addition, the grooves are so designed to remove noise emission caused by the air conducting. On the other hand, the flutings included on the tread bottom work to increase the refrigeration or cooling of the tyre, in addition, flutings contribute to the traction capacity of the tyre. Meanwhile, the shoulders join the tread to the lateral faces; the shoulders are designed taking account of the bending of the lateral faces, resulting in a perfect contact of the tread on the pavement. Furthermore, the shoulders aid the weight transfers over the tyre, when a vehicle passes through curves.
On the other hand, as mentioned above, the second basic structure of a tyre are formed by lateral faces, each one of them having a flange forming an inner ring thereof. The lateral faces of the tyre are attached to the tread by the shoulders and are composed by rubber compounds with high flexural fatigue strength; in addition, the lateral faces may have inside thereof steel cords in order to provide higher strength.
In turn, the flange is formed by ring-shaped steel wires. The wires are copper-coated to avoid oxidation and, in addition, the cords are individually isolated by rubber compounds to prevent friction therebetween. The flange acts to anchor the tyre to the rim, the flange should have a high breaking strength, such that the flange is filled with a hard rubber compound to overcome the severity of the tyre mounting. Upon manufacturing of a tyre, the flanges are attached to the lateral faces while assembling the tyre, and further, they become totally bonded by the vulcanization process.
Once having explained the basic structure of a tyre, it is important to mention that machines allowing to separate the two main parts comprising a tyre are disclosed in the prior art, such as the machine of the Mexican patent 188,444 which belongs to the same inventor of the process being described in the instant document. Said patent is related to a tyre cutting machine, which makes it possible to efficiently separate the lateral faces from the tread, thus decreasing the volume occupied by the used tyres. However, this patent does not mention how the separated parts may be processed in order to obtain homogeneous particles or pellets capable of being recycled with the aforementioned processes and products.
Likewise, belonging to the prior art is the Mexican patent 160,377, disclosing a machine conceived to cut used tyres, said machine incorporates an structure having a cylindrical anvil roller, rotatably mounted thereon. Further, the machine employs, in the structure, a pair of knife rollers that rotate in parallel axes to the anvil roller rotation axis, said knife rollers being angularly spaced apart to each other and to the anvil roller axis, such that the tyres pass through the anvil roller between the knife rollers, thus obtaining rhombus shaped tyre cuts almost having a quadrangular shape. However, like other existing machines, the '377 machine does not take account the structure of the tyre, since tyres are feed to the machine as a whole, resulting in a high power consumption, besides to the fact that the rubber rhombus thus obtained are not uniform, in such a manner that cords and fibers protrude from the rhombus which may harm the personnel working with them.
Another case is that of the Mexican Patent No. 160,376 which describes a similar machine to that in the 160,377 patent, this machine comprises an anvil roller and a knife roller rotating about the axis parallel to the anvil roller rotating axis. Likewise, this machine cut the tyre in pieces without making a difference nor obtaining the maximum benefits from the different parts constituting the tyres.
Another important prior art document, is the Mexican Patent Application No. 9401943, which describes a machine consisting of a pair of platforms to place the tyre wherein the tyre rubber is cut through high pressure fluid cutting operations, however, the inconvenience with this type of machines is that operating a highly pressured fluid is highly expensive, besides to the inconvenience of accomplishing cleaning and recycling of the fluid used in the machine operation.
As may be noted, the prior techniques are limited to cut in a random manner the used tyre, or by means of highly expensive machines, and do not provide a logical sequence to obtain the greatest benefits from the tyre constituents, further, said techniques do not distinguish the particular manner in which the rubber, and polymeric and metal strings are related to each other in a tyre.